The present invention relates in general to the problem of detecting pinking (knocking) in internal combustion engines, particularly with regard to controlled-ignition (Otto) engines for motor vehicles. As is known, pinking occurs in these engines due to the self-ignition of the explosive mixture for various reasons (excess fuel supply, a high compression ratio, certain load conditions, etc.) when the piston is near top dead centre (TDC).
Pinking does not cause substantial variations in the form of the pressure cycle between the compression, combustion and expansion phases of the engine but gives rise to oscillations which are superposed on the normal cycle at its peak.
The frequencies of the oscillations are determined by the dimensions of the combustion chamber and its temperature when the pinking occurs. The typical resonance frequencies of these oscillations can be calculated theoretically with a good approximation, as explained, for example, in U.S. Pat. No. 4,943,776. Since the temperature of the fluid in the cylinders generally increases with increases in the rate of rotation (revs/minute) of the engine, the resonance frequencies may become higher as the rate of rotation increases.
The pressure oscillations generated when pinking occurs are propagated through the walls of the cylinder and can be detected by a sensor. The sensor also detects vibrations due to normal combustion, however, and those due to the moving mechanical members. The signal output by the sensor must therefore be manipulated in order to separate the useful signal attributable to pinking from the background noise which may vary in dependence on the rate of rotation (revs./minute) of the engine and on the extent of wear.
More specifically, pinking can be detected by means of a pressure sensor facing the combustion chamber or by means of an accelerometric sensor, particularly a piezoelectric sensor, on the engine block. The latter type of sensor is usually preferred for reasons of cost and reliability.
In order to detect pinking, the accelerometric sensor is mounted on the engine block, typically on the head or near the inlet manifold.
The signal provided by such a sensor thus includes a noise component and a useful signal component (that is, a component actually linked to the pinking) whose spectrum may have peaks indicative of pinking in a plurality of frequency ranges. Thus, for example, for a cylinder having a diameter (bore) of approximately 80 mm, peaks may be detected substantially at frequencies of 8, 13.8, 15.8 and 18.2 KHz.
The key factor for the correct detection of pinking, therefore, is the establishment of a criterion which enables the useful signal to be separated from the noise component of the signal output by the sensor.
Various devices have thus been proposed and these are analysed and criticised in detail in the introduction to U.S. Pat. No. 4,943,776 cited above.
An improved performance can be achieved by the device which forms the specific subject of that U.S. patent.
The device according to U.S. Pat. No. 4,943,776 achieves a better signal/noise ratio than prior devices by virtue of a channel which evaluates the characteristics of the background noise in a region of the spectrum which is relatively insensitive to pinking. The pinking detection strategy of this device is greatly simplified, however, and presupposes a well-defined and deterministic knowledge of the signal provided by the accelerometric sensor. Moreover, the device does not lend itself to the detection of pinking cylinder by cylinder since the reference signal with which the signal resulting from the processing of the signals in the first two channels is compared represents the average noise of all the cylinders and not that relating to each individual cylinder.
Finally, in the prior device, the strategy for the comparison between the signals processed by the first two channels and the signal processed by the third was quite complex and provided for a preliminary comparison between the signals processed by the first two channels.